1. Field of the Invention
The invention relates to a photoresist having a high proportion of silicon atoms.
Photoresists are used in various ways in numerous lithographic techniques in microelectronics. For example, resists are used as a mask in plasma etching processes in order to structure materials present under the resist, e.g. a silicon wafer. For this purpose, a thin film of photoresist is first applied to the substrate. By exposure to light, which is effected, for example, with the aid of a photo mask, by interference methods or by direct irradiation, for example with an electron beam or a beam of focused ions, chemical differentiation between the exposed and the unexposed parts of the photoresist is produced. After the exposure to light and after any heating step carried out, the parts differ, for example, in their solubility in polar solvents. Chemically amplified photoresists have been developed for achieving exposure times suitable for industrial applications. These contain a photoactive component that liberates a catalyst on exposure to light. For example, a photo acid generator that liberates a strong acid on exposure to light can be added as a photoactive component to the photoresist. A positive photoresist contains a polymer that has, for example, acid-labile groups. By heating (PEB “postexposure bake”), a large number of acid-labile groups can be eliminated after the exposure to light under the catalytic effect of the strong acid liberated. This liberates polar groups, for example carboxyl groups, so that the solubility of the polymer in an aqueous alkaline developer is drastically increased. If the exposed photoresist is developed after the postexposure bake, the exposed parts of the photoresist are detached from the substrate by the basic aqueous organic developer, so that the substrate is bared in these parts. The bared substrate surface can then be processed, for example by dry etching. For the etching, however, the resist structure remaining on the substrate must have sufficient etching resistance. This can be achieved, for example, by a sufficient film thickness of the resist structure.
In order to form even very small structures without imaging errors in the photoresist, increasingly short wavelengths are being used for the exposure. However, the currently available photoresists have a very high absorption at short wavelengths, so that only very thin photoresist films can be used to ensure that, even in the very deep parts of the photoresist film, a sufficient exposure dose is available for liberating the catalyst. In order to achieve sufficient etching resistance, the photoresist structure can be chemically amplified after the development. For this purpose, anchor groups, for example acid anhydride groups, which are capable of reacting with a reactive group of an amplification agent, are provided in the polymer of the photoresist. A suitable reactive group is, for example, the amino group, which can react with the acid anhydride group with formation of, for example, an amide bond. If the dry etching of the substrate is carried out in an oxygen plasma, silicon-containing agents are generally used as amplification agents. The amplification by silicon-containing amplification agents is generally referred to as silylation. During the etching in oxygen plasma, the silicon-containing group is oxidized and a protective film of silicon dioxide forms on the surface of the resist structure, which protects the substrate underneath from attack by the plasma. Such amplification of resist structures is described, for example, in commonly-owned, European Patent EP 0 395 917 B1, which corresponds to U.S. Pat. Nos. 5,234,794 and 5,234,793.
Photoresists in which the polymer already includes silicon-containing groups are also known. These can be prepared by using, for example, allyltrimethylsilane as a copolymer in the preparation of the polymer. Such a photoresist is described, for example, in commonly-owned European Patent Application No. EP 0 957 399 A2, which corresponds to U.S. Pat. No. 6,063,543.
Silicon-containing photoresists have high etching resistance in the oxygen plasma and, as a rule, exhibit high transparency at low exposure wavelengths, such as, for example, 157 nm or 13 nm. During the exposure of the photoresist to light, many bonds in the resist polymer are cleaved by the high energy of the radiation used, in particular at 157 and 13 nm, but to a far lesser extent even at 193 nm. If the silicon in the polymer of the photoresist is contained in a side group, a volatile low molecular weight silicon-containing compound may form after the elimination of the side group. During the exposure of the photoresist to light, expulsion of organosilicon compounds in gaseous form therefore takes place. Because the flushing of the optical systems with nitrogen is as a rule incomplete in the conventional exposure apparatuses, the optical exposure systems gradually become contaminated during the exposure of the resist to light. In contrast to expelled aliphatic and fluorine-containing gaseous products, however, the expelled silicon-containing products decompose slowly to nonvolatile silicon dioxide, which is deposited on the optical systems and irreversibly damages them.